This reference manual explains the function of each subroutine in the Graphics Compatibility System (GCS). GCS is a Fortran-based computer graphics system designed for interactive/passive use a wide variety of graphics hardware. Contained within this section are the detailed descriptions and usage instructions of all of the user callable routines which make up the GCS graphics system, and a discussion of the various options available to the user. GCS is available in two versions, a two-dimensional version, and a three-dimensional version. The three dimensional version is a sophisticated, highly capable package having such features as graphical data structures, text processing, and of course, three-dimension graphics. The two-dimensional version is the older, less sophisticated version. While possessing fewer capabilities, it is still supported, since it offers the benefits of a smaller central memory requirement, and a higher execution speed, while still fulfilling most of the normal graphics requirements. In general, the two-dimension version is a proper subset of the three dimension version meaning that all elements of two-dimension GCS are contained within the three dimensional version.